Mechanism by which Liddle's syndrome mutations increase activity of a human epithelial Na+ channel

A novel splice-site mutation in the gamma subunit of the epithelial sodium channel gene in three pseudohypoaldosteronism type 1 families

Pseudohypoaldosteronism type 1 (PHA1, OMIM 264350) is an uncommon inherited disorder characterized by salt-wasting and end-organ unresponsiveness to mineralocorticoids. A complete genome search using homozygosity mapping in eleven consanguineous families with PHA1 provided conclusive evidence of linkage with heterogeneity. The disease locus mapped to chromosome 16p12.2-13.11 in six families and to 12p13.1-pter in the other five families. These two chromosomal regions harbour the genes encoding the three subunits of the human amiloride sensitive epithelial sodium channel (hENaC): SCNN1B and SCNN1G on 16p and SCNN1A on 12p. Our linkage results have been further supported by the recent report of mutations in the alpha and beta subunit genes in PHA1 patients. We now report the identification of a 3' splice site mutation in SCNN1G (318-1 G-->A) in three families showing linkage to 16p. Abnormal splicing results with the production of two messenger RNAs, one arising from activation of an adjacent cryptic splice site and the other from skipping of the downstream exon. The two corresponding mutant gamma hENaC subunits are predicted to have three highly conserved amino acids in the extracellular domain replaced by a novel amino acid (KYS106-108-->N) and truncation from 649 to 134 amino acids respectively. These three families all originate from the Indian sub-continent and the probands have severe generalized PHA. They share a common haplotype which suggests the presence of a founder mutation in this sub-population.

WW domains

WW domains are recently described protein-protein interaction modules; they bind to proline-rich sequences that usually also contain a tyrosine. These domains have been detected in several unrelated proteins, often alongside other domains. Recent studies suggest that WW domains in specific proteins may play a role in diseases such as hypertension or muscular dystrophy.

Molecular genetics of human blood pressure variation

Hypertension is a common multifactorial vascular disorder of largely unknown cause. Recognition that hypertension is in part genetically determined has motivated studies to identify mutations that confer susceptibility. Thus far, mutations in at least 10 genes have been shown to alter blood pressure; most of these are rare mutations imparting large quantitative effects that either raise or lower blood pressure. These mutations alter blood pressure through a common pathway, changing salt and water reabsorption in the kidney. These findings demonstrate the utility of molecular genetic approaches to the understanding of blood pressure variation and may provide insight into the physiologic mechanisms underlying common forms of hypertension.

Towards prediction of cognate complexes between the WW domain and proline-rich ligands

The WW domain is a structured protein module found in a wide range of regulatory, cytoskeletal, and signaling molecules. Its ligands contain proline-rich sequences, some of which show a core consensus of XPPXY that is critical for binding. In order to gain a better understanding of the molecular and biological functions of WW domains, we decided to predict their cognate ligands by searching databases for proteins containing the XPPXY consensus. Using several axioms that take into account evolutionary conservation and functional similarity, we have identified four groups of proteins representing candidate ligands that signal through known or unknown WW domains. These include viral Gag proteins, sodium channels, interleukin receptors, and a subgroup of serine/threonine kinases. In addition, we proposed that dystrophin and beta-dystroglycan bind through the WW-XPPXY link and that interference with this interaction could result in muscular dystrophy. Our study provides guidelines for experiments to probe the molecular and biological functions of the WW domain-ligand connection. Should these predictions be proven empirically, the results may have important ramifications for basic research and medicine.

Cloning and expression of a novel human brain Na+ channel

We have cloned a novel cDNA from human brain which encodes a non-voltage-dependent Na+ channel (BNC1). BNC1 has some sequence similarity (24-28%) with a new channel family that includes subunits of the mammalian epithelial Na+ channel, the Caenorhabditis elegans degenerins, and the Helix aspersa FMRF-amide-gated Na+ channel. Like other family members it is inhibited by amiloride. However, its predicted structure differs from other family members, its discrimination between Na+ and Li+ is different, and in contrast to other mammalian family members, coexpression with other cloned subunits of the family does not increase current. BNC1 has a unique pattern of expression with transcripts detected only in adult human brain and in spinal cord. Thus, BNC1 is the first cloned member of a new subfamily of mammalian Na+ channels. The function of BNC1 as a non-voltage-gated Na+ channel in human brain suggests it may play a novel role in neurotransmission.